:focus

Arguments: &optional process

With no arguments, print the process on which the listener process is focused.
Otherwise, focus on process, arresting it for debugging if :debugging
is not already on process's list of arrest reasons. Focus on the listener process
if the argument is :listener. Note that the listener process is not arrested.

:focus keeps a stack of processes on which the user has focused. When
the current focus is moved from a process by using the command :unarrest,
this stack is used to determine which process will become the focused process. The
listener will become the focused process if no other processes are on the stack.